Apparatus and method for controlling pulp density



Jan. 1, 1943. H, HARDINGE ,3

APPARATUS AND METHOD FOR CONTROLLING PULP DENSITY Filed Jan. 6, 1939 2 Sheets-Sheet l i 5 a II 6 5 3f F 36 5 25 E 6 w 3/ 23 f 7 I 33 1 w 24 22' 3-1 E Mi 4 E m Q LINE Jan. 19, 1943. HARDlNGE 2,308,917

APPARATUS AND METHOD FOR CONTROLLING PULP DENSITY Patented Jan. 19, 1943 OFFICE APPARATUS AND METHOD FOR CON TROLLING PULP DENSITY Harlowe Hardinge, York, Pa., assignor to Hardinge Company, Inc., York, Pa., a corporation of New York Application January 6, 1939, Serial No. 249,660

21 Claims.

This invention relates to a control system, and more particularly has reference to a method and means for controlling the density of a pulp in a wet grinding mill and also in a classifier if desired.

The importance of closely controlling the density of a pulp in a mill is appreciated, inasmuch as the grinding rate and efiiciency of a mill is a function of the pulp density in addition to other factors. Likewise the desirability of controlling the pulp density in the classifier i well recognized, for upon this density depends not only the eflicient and economic operation of the classifier itself, but the absence of a close control of the density in the classifier will alter the character of product discharged from time to time and also disturb subsequent treatment as well as increase the general cost of operations.

Even after the desired density of pulp is secured in the mill and in the classifier, this density may readily be varied as the result of any one of several factors. For instance, the pulp density will be upset upon a change in specific gravityof the ore or other substance if it is fed to the mill volumetrically. Likewise, changes in the grindability of the ore resulting from variations in the hardness and size of the feed will disturb the density unless the water is varied along with the change in feed rate necessary to take care of such variance in grindability. Variations in the head of the main water supply will, of course, affect the amount of water introduced to the mill or classifier with a given valve setting. And, of course, if the amount of Water supplied the mill and classifier is controlled manually, a change in any one of the foregoing factors may well result in an overcorrection of water supply by the operator. The present inveniion makes provisions for all of the foregoing :auses of density variations, and furnishes means and teache a method of stabilizing the pulp den- ;lty in a grinding circuit.

An object of this invention is to provide a device which may be operated to introduce ore and water to a mill in such proportions as to nsure a substantially uniform pulp density there-- 11 at all times.

Another object of this invention is to provide t system and a method of maintaining substanially constant the density-of pulp in a classifier.

Yet another object of this invention is to proiide a system of controlling the density in a grinding mill or in a classifier, or both, and which ermits of considerable flexibility in operation.

To accomplish the above. and other important all objects as will appear more fully hereinafter, my invention in general embraces the concept of means which may be operated to control the density of pulp in a mill and classifier, and which may be selectively operated to control only one or the other. More specifically, my invention contemplates means for feeding the ore and introducing water to the mill in such proportions and in such amounts as to insure optimum operating conditions in the mill, and furthermore to supply the classifier with water to dilute the pulp therein to a uniform and constant density. In the preferred form of my invention I provide a device for feeding by weight, ore to the mill and means for proportioning a fixed quantity of water, therewith, and further means for supplying additional water to the pulp in the classifier in a predetermined ratio to the water supplied to the mill, said ore and water supplies being controlled in accordance with vibrations emanating from the'mill.

In the accompanying drawings I have disclosed two specific embodiments of my inventive concept, but it will of course be appreciated that 'other apparatus may be used to accomplish the same results without departing from the scope of my invention. In these drawings, in whichsimilar numerals refer to the same parts:

Figure 1 is a system for intermittently feeding ore and water to a mill and classifier in order to maintain constant the operating conditions within the mill and the pulp density in the mill and in the classifier.

Figure 2 is a wiring diagramfor the device shown in Figure 1.

Figure 3 is a view of apparatus in general similar to that shown in Figure 1, but different therefrom in that some ore and water are always being supplied but in which their rate of feed may be varied.

Figure 4 is a wiring diagram for the device shown in Figure 3.

Referring more particularly to Figure 1, I have shown a conventional form of ball mill I which, by means of trough 2- and drum or scoop feeder 3 rotating with the mill, is in closed circuit with a classifier 4 of the so-called rake type. Of course other types of mill and classifier may be used if desired. The ore is fed to the mill through a hopper 5 by means of a feeding device 6 which receives ore from a bin 1 and which is operated by a motor 8. The feeder 6 is of the type which feeds .ore according to weight, as opposed to the so-called volumetric feeder. The motor 8 is adapted to be operated intermittently, by means of a system such as shown inmy co-pending ap plication, Serial No. 249,291, filed January 4, 1939, and consequently the amount of ore actually fed by the feeder 6 is dependent upon the relative "oii and on" periods of the motor 8.

While my invention may be practiced, and stabilization of the pulp density roughly secured when using a volumetric feeder, absolute density stabilization is more nearly approached when a weighing type of feeder is used. This is due to the fact, as above indicated, that the specific gravity of the ore or the volume of a given weight of ore, may change over the course of an operation, which would disturb the optimum grinding conditions in the mill if a volumetric feeder is used.

My invention broadly contemplates a proportioning of water to the ore which is fed to the mill, and might be practiced by interposing a valve in a line connected to a water main, which valve would automatically be adjusted, together with the feeder, in accordance with vibrations emanating from the mill. However, I have found that decidedly improved results are obtained by insuring that the pressure of the liquid ahead of the automatically controlled valve is maintained constant and by providing means to prevent surges that would normally result as the valve is continuously operated.

For instance, referring to Figure 1, I have provided a supply tank 3 which receives water continuously from a line II in which a valve I2 is provided to control the rate of water supplied to tank 3. An overflow pipe 13 is provided for the tank 9 and the valve I2 is adjusted so that, in conjunction with overflow pipe l3, a constant hydrostatic head is maintained in tank 9.

A surge tank 14 is provided and receives water from the supply tank 9 through a line I5. The line I5 is provided with an air dome [6 to prevent water hammer, and is likewise provided with a main metering valve H, and a supplementary metering or cut-off valve I8 which is used when it is desired to by-pass some or all of the water around surge tank I4. Furthermore, an electrically controlled valve l8 which under most conditions is preferably of the solenoid type, is positioned in line I5. However, a motor may operate valve 19 instead of the solenoid, under certain circumstances.

The surge tank I4 is provided with an indicating device, generally designated 2|, to show 33. Both pipes 35 and 36 have their upper ends above the level in the tank and free to the atmosphere, unless the tank is overloaded, and both serve to insure accuracy in proportioning the discharge rate, particularly if the pipe lines 24 and 25 do not discharge at the same elevation.

Adjustment of the metering valve 1 I in conformance with the rate of feed from the feeder 8 is essential to establish the desired ratio of water to solids or pulp density, delivered to and later discharged from the system. Adjustment of valves 2li and 21 are for the primary purpose of proportioning the amount of water for the mill and for the classifier. However, as will be here-. inafter pointed out, these valves are closed to effect a by-pass of either the mill water or the classifier water, or both. Furthermore, the valves 25 and 21 are adjusted, in. conjunction with the valve H, to assure a partially filled tank l4 during all normal operations.

It should also be borne in mind that with the given adjustment of valves I1, 25 and 21, the

head within surge tank I4 will vary in accordance with the operation of feeder 6in other words, as more ore is fed to the /mill, valve 13 will remain open for a longer net period, thus tending to build up the head in surge tank l4, notwithstanding the fact that water is constantly being discharged from the surge tank. But it is to be noted that as the head in surge tank 14 increases, the rate of discharge of water will thereby increase. 4

It will be appreciated that the piping and valves described above permit considerable flexibility in operation of my system. For instance,

as usually preferred, the supply of water for both the mill and classifier may be automatically controlled, and the surge tank interposed for both supplies. In such cases, and assuming of course that the general metering valve I1 is suitably .adjusted, valves 29, 33 and 34 are closed; valves the liquid level within the tank. A pipe line 22 extends from the bottom of the surge tank and is provided with a shut-off valve 23.

The pipe 22 is Td into a line 24, which supplies water for the mill, and to a pipe 25 which supplies water for the classifier. The pipes 24 and 25 are provided with valves 26 and 21, respectively, each of which may shut on completely, or merely meter, the supply of water from the surge tank to its particular pipe.

The water from pipe l5 may be completely or partially by-passed around the surge tank by' means of a line 28 provided at'its upper end with a valve 23. From the lower end of line 28 there are tapped legs 3| and 32 extending to pipes 24 and 25, respectively. Leg 3| is provided with a valve 33, and leg 32 is provided with a valve 34.

26 and 21 are adjusted to establish the desired proportions of mill water and classifier water; and valves i8 and 23 opened wide.

On the other hand, under some circumstances, the use of the surge tank may not be desired for either the mill water or the classifier water. In such case, valves I8, 26, and 21 are closed,

'" valve 28 opened and valves 33 and 34 adjusted to meter the proportionate quantities of water for the mill and classifier. In this way the water for both the mill and the classifier will by-pass the surge tank M.

Then again it might be desired to use the surge tank l4 for the mill water, and by-pass the surge tank for the classifier water. In such case, valves w I8 and 29 will be adjusted to proportion the water to the surgetank and the by-pass line,

valve 26 will be metered to control the water from the surge tank to the mill, valves 21 and 33 will be completely closed, and valve 34 will be opened.

It should be pointed out that the surge tank is of particular value for supplying the classifier water, inasmuch as surges are particularly objectionable in the classifier. On the other hand a by-pass of the surge tank by the mill water possesses some advantages over the use of the surge tank, in that the pulp density in the mill may An overflow pipe 35 extends from the surge be maintained more uniform thereby. Conse tank [4 to the pipe 25 below valves 21 and 34.

This overflow pipe 35 also serves to break the hydraulic leg in pipe 25 below the valves 21 and 34. Likewise a relief pipe 35 serves to break quently, a very desirable operation employs the surge tank for the classifier and a by-pass of the surge tank by the mill water; and in such case, the valves l8 and 29 will be adjusted to proper the hydraulic leg in pipe 24 below va1ves'26 and tion the water to the surge tank and to the bypass line, valve 21 will be metered to control the water from the surge tank to the classifier, valves 34 and 28 will be completely closed, and valve 33 will be open.

And, of course, it may be desired to manually control the water to either the mill or the classiher and to automatically control the water to the other (either using or by-passing the surge tank for the latter). In such a case, manual control is eifected by means of a valve in a separate line (not shown) direct from the main storage tank, and the particular adjustment of the valves for such an operation is believed to be apparent from the foregoing operations.

As indicated above in the preferred form of my invention, the ore feed the mill water, and the classifier water are all controlled in accordance with the sound emanating from the mill. For this purpose, I provide a microphone, designated generally by the numeral 37, which is positioned adjacent the mill, and its preferred construction and location are as set forth in my co-pending application above referred to. The microphone is part of a system generally similar to that shown in the said co-pending application, most of the other elements in said system being housed in a case 38. For a more detailed description of such system and its operation, reference is made to the co-pending application, but the essential features of such system are indicated by the wiring diagram of Figure 2.

The microphone 31 is in the primary circuit 39 of a transformer 4|, in which circuit there is provided a variable resistance 42, and, of course, a source of current 43. In the secondary circuit 44 of the said transformer, there is positioned a relay designated generally 45, a rectifier 46, and an airmieter 41. The relay 45 is adapted to open or close a circuit 48 in which is positioned a time delay switch device 49.

The time delay switch 49 is adapted to make or break a circuit in which is included the feeder motor 8 and the solenoid of the valve IS. The time delay switch, of course, is to prevent fluttering of the current in the last-mentioned circuit, but when the feeder motor and solenoid valve circuit is definitely closed, the motor 8 is set in operation and the valve I9 is opened.

The operation of the control system is briefly as follows: The resistance 42 is regulated so that the system is adapted to operate at the particular noise level of the mill which is found to be started up and the solenoid valve I9 is reopened.

The mill then'tends to become overloaded, the

noise level decreases, and as it drops below the predetermined optimum level, the microphone 31 is de-energized to a point where the first cycle is 5 for a two-speed operation, which is generally repeated.

It will be noticed that the apparatus described above provides for a definite and proportional supply of ore and water to the mill and/or clas- '10 sifler, and, while the system so described is adaptsimilar to the structure shown in Figure 1. For the sake of brevity, I will refer only to those features of Figures 3 and 4 which depart from the disclosure of Figures 1 and 2.

speed motor and in this instance operates at two different speeds depending upon the current supplied to it. Therefore, two different, but deflnite, rates of feeding are secured from the feeder 6.

To provide for the two rates of water supplied to the mill and to the classifier, I split the pipe |5 to form the lines |5a and l5b. In the line [5a is located the valve I9, air dome l6, and a valve 5|. A metering valve 52 is located in line |5b, and

pipes 53 and 54 extend from lines |5a and |5b respectively, to the by-pass line 28.

It will be noted that the lines |5a and |5b unite l to form a line 55 which empties into surge tank 4. In the line.55 there is positioned a valve 56.

[5b. In such a way.

most satisfactory. A detailed description of this step is to be found in my above mentioned copending application.

When the resistance 42 is properly adjusted, and assuming that the noise of the mill falls below the optimum noise level, the current in the primary circuit 39 is decreased with such a resultant decrease in the secondary circuit 44 that relay 45 is opened. This breaks the circuit 48 of the time delay contractor 49, which opens the circuit of the feeder motor 8 and the solenoid l9, thereby stopping the feed of ore and the supply of water to the mill and the classifier.

When the feed and water to the mill is stopped, and as the mill continues to discharge its product, thus tending to underload the mill, the noise level of the mill climbs upwardly. When the noise level reaches the predetermined point of best operating conditions the microphone 37 is sufficiently energized to induce a current in the relay circuit 44 which closes the relay 45 and in turn the time delay switch 49, the feeder motor 8 is microphone 3'! 0 and the valve 29 in line 28 In practice, the two rates of water supply are accomplished in the following manner: Assuming that the water for both the mill and the classifier is to pass through the surge tank I4, is closed and valves 5| and 56 are opened; and, after regulating the general metering valve I1 to give the desired maximum flow; valve 52 is adjusted to provide for a predetermined flow of water through line the surge tank I4 is always receiving at least the predetermined flow through line l5b. This flow is set to provide the requisite quantity of water to maintain the proper pulp density when the motor 8a is operating at low speed.

When the rate of feed of ore is increased, with the consequent demand for an increased water supply, the solenoid valve I9 is opened, permitting water to also flow through line |5a, and thus increasing the supply of water to surge tank H.

The concomitant control of the water and ore supplied to the mill is secured by a control system as shown by the wiring diagram of Figure 4. .In this diagram that part of the system from the to relay 45 is the same as .described above in connection with Figure 2. In the circuit 48, however, the time delay switch 49a is of a double throw type whereby, in its lowered position, it will establish a circuit 51 which includes the motor 8a and also a. resistance 58. This circuit operates the motor 8a at its reduced speed.

However, when the circuit 48 is energized to raise the switch 49a, the circuit 51 is, of course,

broken and there is established a new circuit 59 for the motor 8a in which there is no resistance. This circuit operates the motor 80 at its high speed. It will be noted that, in its raised position, the switch 49a also establishes a circuit Si which includes the solenoid of valve l9, so that,

In the first place, the motor 8a is a variable valve I9 is open.

It will therefore be seen that, when the motor .is running at reduced speed, the water for the mill and classifier is supplied merely through pipe ib. However, when the speed of the motor 6a is increased, water is then supplied through both pipes |5a and b to maintain the desired ratio between the ore feed and the water,

It will be appreciated that the various alternative operations described above with reference to the device shown in Figure 1that is bypassing either or both mill water and classifier water around surge tank I, automatically controlling only one or the other of the mill or classifier water, etc.is possible with the arrangement set forth in Figure 3, by the proper manipulation of the several valves.

The total amount of water admitted to the system is controlled by the general metering valve I1, and the proportioning of the mill and classifier supplies is accomplished by means of sub-metering valves 26 and 21 when both the mill and classifier are supplied from the surge tank. Furthermore, valves 26 and 21, in conjunction with general metering valve l1, regulate the amount or water in surge tank M. If it is desired to avoid the use of such sub-metering valves 26 and 21, I may provide a second line from the general supply tank 9, which is indicated in Figure 1 by the numeral 62, using the line l5, for either the mill or classifier water and the line 62 for the other, or line 15 used for both mill and classifier supplies, and line 62 for a separate regulated supply line to other apparatus.

The line 62 may be provided with a surge tank if desired, but in Figure 1 I- have omitted the showing of this element and provide rather a straight supply line. A metering valve 63 is in terposed in the line 62 to control the amount of water admissible to such line from the tank 9. Likewise, an electrically operated valve 64 is provided, which may be of the solenoid type, and which is operated by the same circuit as that which actuates the valve IS.

The line 62 is T'd into a line 65 which extends to the mill water supply line 24, and which is provided with a valve 66. The opposite branch of the T is a line 61 in which is located a valve 68. A line 69 is Td into the section 61 provided with a valve 1|, the line 69 extending to the pipe for the classifier supply.

It will therefore be seen that the supply of water to the mill and classifier may be secured by the use of two independent systems, so to speak, and by the regulation of the general metering valves l1 and 63, rather than resorting to he various sub-metering valves hereinbefore referred to.

Likewise, I have shown an extension of the line 61 to provide a supply line 12 for other apparatus which treat the material subsequent to the classifier 4, in which apparatus a constant ratio of water to ore may be desired. A metering valve 13 is provided in the line 12 to control the amount of water supplied to such other apparatus.

Of course, if valve 63 is opened wide, then valves 66, 13 and -1| may be used as metering valves ior their respective lines, 65, 12 and 69.

It is believed that a description of the operation of the device shown in Figure 1, with both the mill and classifier water going through surge rank M, will sufiice to illustrate generally the operation of my invention in all of its various alternatives. The mill is started, and the feeder 6 commences to supply the ore, and the solenoid valve I9 is opened. The several valves are closed, which will prevent by-passing the surge tank, valve i1 is partially opened, and valve 23 is completely opened.

The valve I1 is first adjusted to secure the proper density of pulp discharged from the system, or in other words, classifier overflow. The valves 26 and 21 are throttled down so that the surge tank will gradually fill. The density of the pulp in the mill and classifier is then checked to determine the densities in each and if one is too high and the other too low, valves 26 and 21 are altered to suit. If the water overiiows in the pipe 35 quite often, then either or both valves 26 and 21 should be opened somewhat more, so that the level in the surge tank will change as the valve 19 is opened and closed. On the other hand, if the surge tank 14 never fills and actually empties when valve 19 is closed slightly longer than usual, yet not closed an abnormally long time, then valves 26 and 21, more particularly valve 21, should be further closed. In general valves 26 and 21 should be adjusted so that but little water ever overflows through pipe 35 at any time, and one adjusted with relation to the other, so that proper density in the mill is maintained.

It. will be noted that in the case of the on" and off system as shown in Figure 1, the feeder 6 and the valve l1 are adjusted so that, during the on period, ore and water are fed at rates in excess of that necessary to maintain the desired noise level if the feed were continuous, instead of intermittent. Likewise, in the case of the multiple speed system of Figures 3 and 4, the higher rate of feed is in excess of that necessary to maintain the desired noise level. However, the average rate of feed over a given period necessary to maintain the desired noise level is secured by balancing the on periods against the off periods in the case of the intermittent operation, and by balancing the higher speed against the lower speed in the case of the multiple speed operation.

In summary, it willbe noted that I have provided a system in which the density of the pulp discharged by the classifier will remain substantially constant as an average, over a given period. Likewise, I have provided means for maintaining a substantially constant pulp density within the grinding mill which, of course, is of a different degree from that obtaining in the classifier. It will be noted that my system possesses considerable flexibility and is designed to be altered to meet varying conditions of operation. Its simplicity of construction and the economies effected by its inclusion in a milling circuit render it a valuable addition to any mill.

While I have shown several specific embodiments of my invention, it will be appreciated that various other alternative forms are embraced by my inventive concept, the scope of which is to be determined solely by the appended claims.

I claim:

1. In a method of controlling the pulp density in a wet grinding process, the steps of determining the desired noise level in the grinding 1y greater than and less than those necessary to maintain the desired noise level, and con-' trolling the duration and frequency of such periods to provide an average supply that will,

insure the desired noise level.

2. In a method of controlling the pulp density in a wet grinding process including a mill and a classifier, the steps of determining the desired noise level in the grinding apparatus, determining the pulp density necessary for such a and less than those necessary to maintain the desired noise level, controlling the duration and frequency of such periods to provide an average supply that will insure the desired noise level, and simultaneously with such supplies to the mill providing the classifier with water in additlon to that in the pulp normally introduced into the classifier whereby substantial uniformity of the pulp is secured. i

3. In a method of controlling the pulp density in a wet grinding process including a mill and a classifier, the steps of determining the desired noise level in the grinding apparatus, determining the pulp density necessary for such a noise level, thereafter simultaneously supplying feed and water to the circuit in a definite ratio to substantially maintain the desired pulp density and in quantities periodically greater than and less than those necessar to maintain the desired noise level, controlling the duration and frequency of such periods to provide an average supply that will insure the desired noise level, simultaneously with such supplies to the mill providing the classifier with water in addition to that in the pulp normally introduced into the classifier whereby substantial uniformity of the pulp is secured, and controlling the introduction of such additional water to the classifier to prevent surges therein.

4. A system for controlling the density of pulp in a grinding circuit comprising a mill, means for supplying solid material by weight to the mill, means for supplying a liquid to the mill in a definite proportion to the solid material, and means for regulating both supply means in accordance with vibrations emanating from. the

50 regulated so that the head is maintained in the mill.

5. A system for controlling the density of pulp in a grinding circuit comprising a mill, means for supplying solid material by weight to the mill, means for supplying a liquid to the mill in a definite proportion to the. solid material, means for regulating both supply means in accordance with the noise level of the mill, and means for diminishing surges of the liquid in the grinding circuit said means comprising a tank into which the regulated liquid supply is delivered, a controllable outlet valve regulated in accordance with the amount of liquid delivered to the mill to maintain a partial filling of said tank, said amount of liquid varying with the pressure head of the tank.

6. A system for controlling the density of pulp .in a grinding circuit comprising a mill, 2. classifier, means for supplying olid material by weight to the mill, means for supplying water by volume to the mill, means for supplying additive water by volume to the classifier,.and means for regulating the material and water supplymeans in accordance with the noise level of the mill.

7. A system for controlling the density of pulp fier, means for supplying solid material to the mill,.means for supplying a liquid to the mill at a definite ratio to the material supply, means for supplying additive liquid to the classifier in a definite proportion to the solid material supplied to the mill, means for regulating the material and Water supply means in accordance with the noise level of the mill, and means for diminishing surges in the water supplied to the mill and the 19 classifier, said means comprising a tank into which the regulated liquid supply is delivered, a controllable outlet valve regulated in accordance with the amount of liquid delivered to the mill to maintain a partial filling of said tank, said amount of liquid varying with the pressure head of the tank.

8. A system for controlling the density of pulp in a grinding circuit comprising a mill, a classifier, means for supplying solid material to the a definite ratio to the material supply, means for supplying additive liquid to the classifier in a definite proportion to the solid material supplied to the mill, means for regulating the material and Water supply means in accordance with the noise level of the mill, means for diminishing surges in the Water supplied to the mill and the classifier, and means for by-passing the mill liquid or the classifier liquid around the surge v diminishing means.

9. A system for controlling the density of pulp in a grinding circuit comprising a mill, means for intermittently supplying solid material to the mill by weight, means for supplying a liquid to the '35 mill in a definite relationship to and simultaneously with the supply of solid material to the mill, and means for regulating both supply means in accordance ,with the noise level of the mill.

10. A system for controlling the density of 10 pulp in a "grinding circuit comprising a mill,

means for supplying solid material to the mill at diiferent rates, means for supplying a liquid to the mill at different rates and in a definite ratio to the supply of solid material to the mill, and 5 means for regulating both supply means in accordance with vibrations emanating from the mill and means intermediate the liquid supply means and the mill, said means comprising a tank with a controllable outlet valve said valve being when the feed and liquid rate increases until the flow from the outlet valve equals the liquid .feed to the tank and a decrease in feed rate will lower the head in the tank and decrease the liq- .uid supply in direct ratio to the solids ted.

11. A system for controlling the density of pulp a grinding circuit comprising a mill, a' classifier adapted to receive the discharge products from the mill, means for intermittently supplying solid material to the mill, means for intermittently supplying a liquid to the mill and a. liquid to the classifier at a definite ratio to and simultaneously with the solid material supplied to the mill, and means for regulating both supply means in accordance with the noise level of the mill and means intermediate the liquid supply means and -the mill, said means comprising a tank with a controllable outlet valve said valve being regulated so that thehead is maintained in the tank whereby any variation in the rate of solid feed in a grinding circuit comprising a mill, a cla'ssip p at q d feed rate to the mill mill, means for supplying a liquid to the mill at varies the head of the tank in direct ratio thereby increasing the liquid head in the tank when the feed and liquid rate increases until the flow from the outlet valve equals the liquid feed to the tank and a decrease in feed rate will lower terial to the'mill, means for supplying a liquid to the mill, and means operable in accordance with the noise level of the mill for supplying additive liquid to the classifier, said additive liquid being fed through a surge tank, said tank being provided with a regulatable outlet valve to maintain a head of liquid in the tank.

13. A system for controlling the density of pulp in a grinding circuit comprising a mill, a classifier adapted to receive the discharge products from the mill, means for feeding solid material by weight to the mill, means for supplying a liquid. to the mill and to the classifier in a definite ratio to the weight of the solid material supplied to the mill, a surge tank intermediate the liquid supply means and the mill and classifier, and means operatively connected with the solids feeding means to vary the hydrostatic head in the surge tank,

14. A system for controlling the density of pulp in a grinding circuit comprising a mill, a classifier adapted to receive the products of discharge from the mill, means for feeding material by weight to the. mill, means for supplying a liquid to the mill, independent means for supplying additive liquid to the classifier, the solid material supply means and both liquid supply means being operatively connected and being responsive to variations in the noise level of the mill.

15. A system for controlling the density of pulp in a grinding circuit comprising a mill, a classifier, means for supplying solid material by Weight to the mill, means for supplying a liquid to the mill in a definite proportion to the solid material supplied, means for supplying a liquid to the classifier in a substantially definite proportion to the solid material, means for diminishing surges of the liquid supplied to the classifier, and means v treating apparatus.

17. A system ior'controlling the density of pulp in a grinding circuit comprising a mill, a classlfier adapted to receive the discharge products of the mill, means for supplying solid material to the mill at different rates, means for supplying a liquid to the mill and classifier at difierent rates and in definite ratios to the supply of solid material to the mill, and means for regulating both supply means in accordance with the noise level of the mill, a surge tank, the supply of liquid to the classifier being directed through said surge tank, which tank includes a regulated outlet valve to maintain a head in the tank, said head varying with the rate of liquid fed, and a second liquid supply means to the mill, said last mentioned means being controllable so as to obtain a definite ratio of liquids to the solids fed,

18. In a system for controlling the density of pulp in a wet grinding mill, a source of solid material to be ground in the mill, means for feedingsolid material for controlling the quantity of liquid supplied to the mill, said last mentioned means including means responsive to sound producing vibrations set up in the mill.

19. A system for controlling the density of pulp in a grinding circuit comprising a mill, means for feeding solid material to the mill, means for supplying a liquid to the mill, and means responsive to the feed of one of the constituents to control the feed of the other proportionately, said control means including means responsive to sound producing vibrations produced in the mill.

20. Asystem comprising a grinding mill which discharges varying quantities of solids, apparatus for treating the products discharged from the mill, a source of liquid to be fed to the treating apparatus, and means responsive to sound producing vibrations emanating from the mill operatively connected to the liquid supply means and controlling the amount of liquid supplied to the treating apparatus to maintain substantially constant density of pulp in the said apparatus.

21. A system comprising a grinding mill which discharges varying quantities of solids, a classifier adapted to receive the discharge products from the mill, apparatus for treating the overflow from the classifien'a source of liquid to be fed to the treating apparatus, and means responsive to sound producing vibrations emanating from the mill operatively connected to the liquid supply means for the treating apparatus and controlling the amount of liquid fed to such apparatus to maintain constant density of pulp in the said HARLOWE HARDINGE. 

